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Unknown Lamer
on Monday November 19, 2012 @06:50PM
from the one-way-trip dept.

ananyo writes with bad news for John Titor. From the article: "Four years after its closure, researchers working with data from the Stanford Linear Accelerator Center's particle physics experiment BaBar have used the data to make the first direct measurement confirming that time does not run the same forwards as backwards — at least for the B mesons that the experiment produced during its heyday. The application of quantum mechanics to fundamental particles rests on a symmetry known as CPT, for charge-parity-time, which states that fundamental processes remain unchanged when particles are replaced by their antimatter counterparts (C), left and right are reversed (P), and time runs in the reverse direction (T). Violations of C and P alone were first seen in radioactive decays in the 1950s, and BaBar was used to confirm violations of CP in B meson decays in 2001. To keep CPT intact, that implies that time reversal is also violated, but finding ways to compare processes running forward and backward in time has proven tricky. Theoretical physicists at the Universityof Valencia in Spain worked with researchers on BaBar to exploit the fact that the experiment had generated entangled quantum states of the meson Bzero and its antimatter counterpart Bzero-bar, which then evolved through several different decay chains. By comparing the rates of decay in chains in which one type of decay happened before another, with others in which the order was reversed, the researchers were able to compare processes that were effectively time reversed version of each other. They report in Physical Review Letters today that they see a violation of time reversal at an extremely high level of statistical significance."

Physicists on the CPLEAR experiment will be disappointed as well - they actually discovered this effect (called T-violation) back in the 1990's before Babar was running by looking at kaon oscillations produced in low energy proton/antiproton collisions [Phys. Lett. B 444 43 (1998)]. So Babar was certainly not the first experiment to see the "arrow of time" although it is the first to do so using B mesons.

From TFA: "Measurements indicating time reversal was likely violated had already been made in kaons at Fermilab in Batavia, Illinois and at CERN near Geneva, but in those experiments, according to Anulli, the measurement of time reversal were not disentangled from violations of charge-parity that were also present."

I've been disappointed since I realized the books were very engaging, exceptionally self-consistent, and... not only sexist but was a central component of the story. That sorta ruined it for me.-- A sad panda geek girl.

Sexist? C'mon, seriously? Women were construed in a particular light were they? But, so were men? Granted, Robert had some funny views and an odd angle to things, but both sexes were firmly put in boxes. No need to get your knickers in a knot.

>Hardly. Recognizing that there are differences between the sexes is common sense. Judging people's values and defining their roles solely by their sexes is sexism.

Those "differences" are provably superficial and purely cultural -as well proven by other cultures having radically different role expectations from sexes and both sexes playing into those different expectations with the same level as they do to the ones in ours.Hell even in our own subcultures this view is greatly challenged. Consider for example the highly androgynous gender-roles of the metal and goth cultures - or the outright girly look of glam metal (which then ironically became equated with having the "guts to be glam" - the MOST masculine thing a man could do was to act feminine).

No my friend, the differences between the sexes exist only in the physical form.

>It's foolish to ignore either aspect. Watching my wife after she had our child, there are *definitely* strong biochemical and physical differences between men and women.

I dispute both claims. And the idea that there is only 2 sexes, let alone only 2 genders. The evidence to the contrary is too strong.For your own example: are you aware that newborns recognize their FATHER's voice BEFORE they recognize their mother's voice ?If you see "different biochemical reactions" in her - I call that her being more

That is what you want to believe. Unfortunately for you that is not the truth. Obviously there are cultural aspects that influence roles, but many of these cultural aspects are stemmed on physical differences and even in the absence of culture experiments have proven time and again that men's and women's instincts and thought processes have distinct differences. There is no motive to take offence on that as different is not necessarily better or worse.

No. Ever been married? A woman seems incapable of putting anything where they picked it up from, and every single married man I ever met concurs. "Where's the stratodoober, dear?"

"Just LOOK for it!" she says. It goes all the way back to hunter-gatherer societies, where women would pick fruits and nuts and berries, and men would hunt game. Evolutionary pressures made women adept at finding stationary things, and men adept at seeing movement. Tha

Whan read backwards it sounds like a foreign language and I can't understand a word. Thus, no change in that.

Now, seriously, the paper's abstract makes more sense than the article. And it is heavy in a jargon that I don't completely understand, while the article was arguably translated into normal english. What a bad translation!

In theory the basic mathematics of quantum theory is time-symmetric. You can write equations to describe particles x and y colliding to produce a and b, and those equations work perfectly well to describe particles a and b colliding to produce x and y. It's why Feynman diagrams are so useful, you can just flip the time dimension around and see something else described by the same maths.

The point of what these folks have done is to look very closely at one particular Feynman diagram, that of the B meson decay, and showing that it is not time symmetric in some way. So the flow of time is something extra on top of the basic quantum theory...that's fascinating.

They would be more specific about the arrow of time. I get that they have confirmed it and all, but which direction is it pointing?

As I understand it (greatly simplified), time is a consequence of matter and energy interactions in space; They don't all happen at once though because of separation, and the distance (relative or absolute) between them is what creates time. That's why they call it 'spacetime'; The smallest unit of time then is the fastest change in quantum state possible. As time is a byproduct of matter and energy interactions, and couldn't exist without it, there's still the question of the "arrow of time". We perceive it to be always moving "forward", but there's no reason why the reaction A-B-C shouldn't go C-B-A as well, or instead.

If I understand this experiment correctly, what they're saying is "as well" is bogus. It's not just that it isn't observable, but that it just doesn't happen. No matter which way the reaction chain goes, there's no mirror reaction that goes unobservable. But perhaps someone who actually is a particle physicist could provide a layman explanation better than mine... I'll be honest: Most of what they do is beyond my grasp because they talk mostly in math and seem to eschew visualization or story explanation. -_-

The arrow of time is the reason why random bits of shrapnel and chemicals don't fly together and "un-detonate" to become hand grenades. In one direction of time, entropy in the universe always increases; in the other, it always decreases. The question is, why? If everything at the quantum level always worked the same way forwards as it does backwards, then entropy would be constant; the universe would be in some kind of steady state and nothing would matter because we wouldn't be here.

I think at this stage of research, it's more about finding clues than it is about trying to put them together into a coherent explanation. But if that's not true, I'd love to hear from someone who really knows this stuff..

The question is, why? If everything at the quantum level always worked the same way forwards as it does backwards, then entropy would be constant; the universe would be in some kind of steady state and nothing would matter because we wouldn't be here.

I don't really know this stuff that well. I did well in quantum computing, which covers the philosophy of the quantum postulates, but never really gets past baby level physics. That said, I've been thinking that maybe there's a bit more to flip than C, P, and T to get a better picture.

If you think of an explosion starting at time 0 and centered at [0,0,0], you start with density concentrated around that point and over time it spreads out from that point. Alternatively, starting at time t and going backwards

If everything at the quantum level always worked the same way forwards as it does backwards, then entropy would be constant; the universe would be in some kind of steady state and nothing would matter because we wouldn't be here.

That's not true. "Everything at the quantum level always working the same way forwards and backwards" is completely consistent with the second law of thermodynamics ("entropy never decreases"), and completely consistent with the observable universe (barring CP violation). All that's necessary is that the universe started with very low entropy -- like, say, the Big Bang.

See for example this from this Arrow of Time FAQ [preposterousuniverse.com] (from cosmologist Sean Carroll):

The observed macroscopic irreversibility is not a consequence of the fundamental laws of physics, it's a consequence of the particular configuration in which the universe finds itself. In particular, the unusual low-entropy conditions in the very early universe, near the Big Bang. Understanding the arrow of time is a matter of understanding the origin of the universe.

The arrow of time is the reason why random bits of shrapnel and chemicals don't fly together and "un-detonate" to become hand grenades. In one direction of time, entropy in the universe always increases; in the other, it always decreases. The question is, why?

The reason is very simple. Entropy is a measure of the probability of a particular outcome. The statement that "entropy increases" is simply the statement that the most probable thing to do happen is almost always the one that does happen. The "almost

The arrow of time is the reason why random bits of shrapnel and chemicals don't fly together and "un-detonate" to become hand grenades.

No, that is entropy. The reason that balls fall off tables and rarely bounce onto them (when provided with enough heat energy) is because there are many, many more states where the balls atoms vibrate incoherently and only one state (or a tiny handful) where the vibrations are organized enough to cause it to bounce back onto the table.

With mesons you can study a particle oscillating between two states. What you find is that the P(A -> B) is not equal to the P(B -> A) where B is the anti-particle state of A and there is no entropy involved. It's all to do with something called CPT symmetry which is a result of relativity and, since CP together are violated (anti-matter is not exactly the same as matter) we expect that T (time reversal symmetry) is also violated so this is an expected result.

FTFA, what makes the difference here is the measurement was made independantly from the CP violation. Here is the key passage:

“It was important to measure time reversal independently of charge-parity violation because there was always the possibility something was wrong with the full picture,” says Fabio Anulli of the National Institute for Nuclear Physics in Rome, who is physics coordinator for BaBar.

Measurements indicating time reversal was likely violated had already been made in kaons at

The arrow of time refers to the fact that we perceive a difference between the past and the future: we remember the past, but not the future. That's explained adequately by noting that entropy tends to increase and the universe, for some reason, was in a low entropy state in the past.

What they've found is that, at least for b-mesons, going forward in time is different than going backward in time, presumably in addition to the rest of the universe accumulating entropy. It's as if there was a fundamental difference between moving "north" and moving "south" in empty space.

The arrow of time refers to the fact that we perceive a difference between the past and the future: we remember the past, but not the future. That's explained adequately by noting that entropy tends to increase and the universe, for some reason, was in a low entropy state in the past.

What they've found is that, at least for b-mesons, going forward in time is different than going backward in time, presumably in addition to the rest of the universe accumulating entropy. It's as if there was a fundamental difference between moving "north" and moving "south" in empty space.

It's as if there was a fundamental difference between moving "north" and moving "south" in empty space.

Would a ratchet favoring one direction serve as some kind of applicable metaphor, or is it too crude to make any sense at all?

I understand well the entropy side of things, in that, since the universe started at a low entropy (an unlikely configuration) it moves towards high entropy over time (toward more common configurations.)

Would it be worthwhile to think of the T violation (in terms of visualization mi

As far as the laws of physics are concerned, you can "move" either way in time and it doesn't make any difference (except for small violations like this one). It's just that you only remember going one way. For some reason we remember "back" in time, when entropy was lower.

Take a video of some physical process and run it in reverse. At a macro level we humans can generally tell that the video is in reverse - tea cups break apart when hitting a floor, they don't spontaneously assemble and then fly off the floor. However, if you analyze such situations using the physics you learned in high school, there is no way to tell that the course of events has been reversed - statistically it's very unlikely that a tea cup would do that, but there is nothing physically impossible about it. So it appear that the laws of physics are the same if time was running in reverse yet to us humans it does not appear that things would be the same if time was running in reverse - because of entropy.

This is the problem of the arrow of time - how can we tell in a physical way which way time is running? Is there any way to distinguish going forward in time to going backward in time using just physical laws? You could say that entropy increases with time (the basis of how we humans can tell on a video whether it is running forward or backward), but that is only a statistical observation and it only holds because it just happens to be that our past has a very small amount of entropy compared to the high entropy situation that the universe will eventually reach. Increasing entropy is a consequence of an accident of what our past looks like and it is not a physical law in the strict sense we are looking for here. So entropy is not a good candidate. This research shows a way that you actually CAN tell if time is running in reverse. Though physicists still believe that there is a CPT symmetry, indicating that if you reverse time and also two other things, then there is no way to tell from physical laws that you did that.

Boomerangs have an airfoil shape in cross section, bananas are just circular in cross section. its the lift generated by the airfoil shape, and gyroscopic forces that cause a boomerang to come back. I wonder how many millenea it took for the aborigines to refine that

Same. I hope some quantum physicist will chime to mention how one can observe time going "backwards" and how this extremely high level of statistical significance isn't another way of saying that they can't.

The really powerful arrow of time is the thermodynamic one. The second law of thermodynamics says that entropy always increases. This thermodynamic arrow is essentially the same arrow as the psychological one, which allows us to remember the past but not the future, and all the other ones we see in nature, such as the laws of black hole thermodynamics, which say that the area of a black hole's event horizon always grows with time. This group of time-arrows, which are all essentially the same time-arrow, appear to occur because the big bang was fine-tuned to be extremely low in entropy, with its gravitational-wave degrees of freedom inactive. Nobody knows why we had a low-entropy big bang, when a random choice of initial conditions would be overwhelmingly more likely to produce a maximum-entropy one. (In particular, inflation doesn't explain it. Also, statistical mechanics doesn't explain it, because to produce the second law from statistical mechanics, you need to assume a low-entropy initial state.)

This paper is about an arrow of time that is obscure and completely unrelated to the others. It has to do with the weak nuclear force. Unlike the others, it has essentially no effect on the world we see around us.

Well, except for being necessary for CP violation, which in turn is the only way we have of explaining why there isn't much antimatter around.

So it does explain why the planet is here and doesn't experience nuclear-style detonations many times an hour as antimatter grains of dust hit the atmosphere. But other than that no effect on the world around us.

Could it just be that all universes exist but since virtually all the other kinds from ours don't produce life, the odds are very high that we exist in one with a "tuned" big bang?Kind of a cross between the anthropic principle and that recent theory I remember saying that all universes based on consistent logics exist..

The summary is a bit confusing if you don't know what it's talking about. The title is even worse, since it implies the exact opposite of what it actually means. Let me try to explain it.

First: physicists believe that the "arrow of time" isn't a fundamental property of the laws of nature. There's no fundamental difference between "forward in time" and "backward in time". The laws of physics operate identically in both directions. So why do those directions seem so different? Why do objects fall down but not up? Why can you make an egg into an omelet, but not an omelet back into an egg? Why can you remember the past, but not the future? This turns out to be a property of our local region of spacetime. More precisely, we live very close (a mere 13.5 billion years or so) away from a point of incredibly low entropy (known as "the big bang"), and that creates an entropy gradient throughout our region of spacetime. What we call "forward in time" simply means "the direction of increasing entropy", or more simply, "away from the big bang".

A good analogy (not involving a car - sorry!) is the direction "down". It seems obvious to you that one particular direction in space is fundamentally different from all other directions. Objects fall down. They don't fall in any other direction. Yet to person on the other side of the earth, the direction they call "down" is completely different from the direction you call "down". That's because the "arrow of gravity" is not a fundamental property of the laws of nature, just a property of our local region of space. "Down" means "toward the center of the earth." In the same way, "forward in time" means "away from the big bang".

Second: what I just said swept a few details under the rug. You see, the true symmetry is not time reversal (which would imply that simply reversing the direction of time would leave all laws of physics unchanged), but a slightly more complicated symmetry called CPT invariance. That stands for Charge, Parity, and Time. It says that if you multiply the charge of every particle by -1 (so positive charges become negative and negative become positive), flip space as if in a mirror so that your left and right sides are reversed (a "parity inversion"), and reverse the direction of time, then all the laws of physics are left unchanged.

Scientists had previously observed a violation of CP. That is, swapping only charge and parity is not an exact symmetry of the universe. If CPT is an exact symmetry (which scientists generally believe), that implies that T is not - changing only the direction of time without also swapping charge and parity should change the laws of physics. But testing that experimentally turned out to be very hard to do. Well, they've finally done it. And the results are exactly what people expected: it appears that CPT really is an exact symmetry of the universe.

In a Newtonian universe, light will follow the same path backwards if it's direction is reversed (bounced perfectly normal to a mirror). My question is "does this hold under relativity?". I thought the answer was yes, but IANAPhysicist so don't know if that's the accepted answer. If it does hold then there are some very interesting consequences that are never talked about.

Photons moving through a medium are "slowed down" by interactions of the electromagnetic field with the atoms of the medium.

Remember that a photon is just localised electromagnetic energy. In a medium, the electromagnetic fields behave differently than in a vacuum, because of the all the atoms with their various charged bits (protons, electrons) -- there is a different "resistance" to changing the field strength because the field has to move the atoms as well. This resistance to changing the field strength

I think you misunderstand. Light only ever travels at the speed of light.However when in a medium (such as water or glass) the interactions of light with the medium (by absorption/re-emission) cause the effective group velocity of the light to be slower.One photon could pass through the block of glass and not interact with any atoms at all and therefore travel at the speed of light. However it is vastly more probable it will hit a silicon or oxygen atom and excite an electron which will then re-emit the pho

Yes, and not just photons. Any particle will follow the same path backward, as long as you also reverse its charge (which has no effect on a photon, since they're uncharged) and parity (which I think flips the polarization of a photon, but don't quote me on that). What CPT invariance really says is that there are two ways of describing the universe that are exactly equivalent in every way. They predict exactly the same result for any experiment you can ever do. But what one description calls "forward in

So if CPT is the true symmetry of the universe, then doesn't that solve the matter/antimatter problem (the problem that we observe more matter then antimatter)?

Say that the Big bang didn't create 'just' our universe, but a second that is a complete CPT reversal from ours. That universe would have the same kind of physics but with the arrow of time reversed relative to ours and would consist primarily of antimatter with minute quantities of matter. And 'left' probably being our universe equivalent o

Physicists tend not to speculate about things that can't be rigorously pinned down with math or in a lab experiment. At least not publicly. Non-physicists generally don't know what they're talking about when they discuss physics. And all people tend to overlook or dismiss experiential data points that lie outside of their existing mental models of how the world works - they don't know what to do with it.

Attempts I've seen to explain or lend credibility to religious or paran

I think you raise a good point that physicists tend to follow only math or lab results, this leaves out the other important aspect of physics, philosophy. So many people think they sit in opposition, physicists should treat philosophy as some kind of irrational mysticism. Truth is, its very easy to miss logical fallacies in experiment proposals because you might think that since anything is possible, any experiment is a valid one. If you include philosophy into the design of a physics experiment, you get to

Conclusion without any need for experiments: If you believe reality is consistent, time does not travel in 2 directions in 1 universe.

We've never managed to go without need for experiments. It's worth noting here that there is an odd sort of higher order of empiricism over general philosophy. One can observe systems which model philosophical arguments. We are such. But there aren't purely philosophical arguments which say concrete things about reality. One needs empirical priors.

And yet, so many physicists have no idea how important self-consistency is in their physics.

You base this on what? Your claims about assumptions on time are addressed both by actual observation of the dimension of time (confirming the validity of the space-time model at the scale in which it is used) and by the procedure of change of coordinates (which provides the consistency you claim is missing from discussion of time).

If you believe reality is consistent, time does not travel in 2 directions in 1 universe.

Actually you raise an interesting point. If there are no CPT violations then time could travel in both directions and we would be none the wiser.Imagine the last few seconds I just experienced suddenly ran backwards - without CPT violations I would be back at the same initial starting conditions and then time goes forwards again and I'd have exactly the same outcome. Time could move forwards and backwards totally randomly and we would never be any wiser.With CPT violations of course that could not happen.

We don't have to assume it. We can observe the dimension of time with a clock. If we start moving the clock a bit then we change the axis which the clock is measuring. This indicates that the dimension of time exists, but isn't unique due to how we can change frame of reference.

it could this far along and a sequence of actions moved it in this direction, reversing all the actions perfectly moves everything the other way.

Sequence of actions isn't well defined. It is quite possible to observe different sequences of events based on where you are and how fast you are moving. One event follows another only if the second event is in the future cone [wikipedia.org] of the

Please, I think you are both too small and too slow to see it for what it really is when you look at a clock. You assume it is a dimension as we all do, since for our scale of things, this works pretty well.

> No, it's not. Something is static if it doesn't change with respect to time. Any parameter changes with respect to itself at rate 1.

Again, please I refer to time "itself" to try and catch you before you trip. When I say in this circumstance, ti

Please, I think you are both too small and too slow to see it for what it really is when you look at a clock.

Not at all. Watching a clock is something everyone can do. Evidence trumps philosophy.

You assume it is a dimension as we all do, since for our scale of things, this works pretty well.

And I'm right too. And yes, you may be right as well about smaller (Planck) scales not having a time dimension.

When I say in this circumstance, time "itself" is static, I mean to say that if it can be reversed along with physical actions and you get back what you started with, then it is independent and unchanging

With respect to what? Static implies constant with respect to time which is itself. But it's not. Predetermination or its absence would not change that.

This isn't at all a problem. After all, we can already observe in every unit of space, time moving forward or backwards as we see it.

No. I'll stop here. This is just wrong, so I won't go any further with it.

Why stop here? There may be some part of the universe which doesn't have a time coordinate, but we don't observe it. What we do observe can have time moving forwar

It actually is very well defined. The light cone is not about sequence of events but sequence of IMAGES of the events you see. The actual order of events in which they actually happen is constant for any observer. The fact that they don't see its image yet doesn't mean it didn't happen yet.

The fact that you don't see the light of an event doesn't mean it didn't happen.

As I note, you aren't in a situation to determine whether an event happened or not until that event passes into your past light cone. A big point to make at this time is that there are places and frames of reference where your current state and the event can be observed to happen in either order. These always exist.

Hence, it is incorrect to state that the event has or hasn't happened yet. It can end up happening before or after your current state, depending on some observer's point of view.

A symmetry under X means the system under test is unchanged (ie the same physical laws work, your predictions are still correct) when you do X.

A simple example is the symmetry under spatial translation -- if your experiment still behaves the same way if it's moved a meter to the left, it has "spatial translational symmetry". This symmetry isn't exactly true on the surface of the earth because of variations in the gravitational field etc., but on a small scale for lab experiments it's true, and in deep space

After detecting and identifying the mesons, the experimenters determined the proper time difference between the decay of the two B states by determining the energy of each meson and measuring the separation of the two meson decay vertices along the e-e+ beam axis. When time-reversed pairs were compared, the BaBar collaboration found discrepancies in the decay rates. The asymmetry, which could only come from a T transformation and not a CP violation, was significant, being fourteen standard deviations away from time invariance. Thus the long wait for an unequivocal time-reversal violation in particle physics is finally over.

IANAP, but here is my understanding of the experiment. They knew that two different decay chains occur from some positron/electron collisions. If time is symmetric, there should be equal numbers of both chains. By making the beam energies different between the positron and electron (e-e+) beams, they were able to differentiate the decay order. If time symmetric decay occurred then there would be one spacial pattern in the results, and if time was asymmetric there would be another. The results conclusively show that for this subatomic event time runs in the direction we know as "forward". This is a big deal for subatomic physics.

Let me explain. Most reactions are time-reversible. (Sort of) example: oxygen and hydrogen can combine to form water and release energy, but you can put energy back into the system to get hydrogen and oxygen back out again (thermodynamics states you will always lose some energy in this process, however, no matter how efficiently you conduct the H+O->water->H+O process).which indicates time is not perfectly reversible, but doesn't explain why). At the subatomic level, however, some similar (vaguely similar, anyways) reactions cannot be reversed, or don't reverse in the same way. In this case, they studied a meson that spontaneously changes from matter to antimatter and back (don't ask). If time reversibility held true for them, the probability of matter->antimatter would be the same as the probability of antimatter-> matter. It was not, by a very very very very significant margin (14 sigma, or a 1 in 10^43 chance this was seen by accident). Note this may also help to explain why matter is more prevalent than anti-matter in our universe.